Monoclonal antibodies (mAbs) are widely used as targeted therapies for the treatment of human malignancies. However, despite the impressive responsiveness of certain types of cancers, resistance to many mAb therapies remains a serious clinical challenge for ovarian cancer, the most lethal gynecologic malignancy. A key activity of many therapeutic mAbs is their induction of antibody-dependent cell-mediated cytotoxicity (ADCC) by natural killer (NK) cells. Human NK cells exclusively recognize tumor bound antibodies by CD16a (Fc?RIIIa/FCGR3A), and its expression levels directly correlate with ADCC potency. CD16a expression and NK cell cytotoxic function, however, can undergo a marked down-regulation in cancer patients, including those with ovarian cancer. Thus, blocking CD16a down-regulation has clinical significance. CD16a down-regulation occurs by a proteolytic process upon NK cell activation by various stimuli (e.g., engagement of IgG, pro- inflammatory cytokines, and tumor cell interactions). We were the first to report that the membrane metalloprotease ADAM17 is expressed by NK cells and is the primary protease that cleaves CD16a. Moreover, we have determined the ADAM17 cleavage site in CD16a and have expressed a non-cleavable yet functional version of the receptor in NK cells. We hypothesize that blocking CD16a shedding in NK cells will augment their killing of ovarian cancer cells in the presence of therapeutic mAbs that induce ADCC. To test our hypothesis, we have devised distinct approaches to inhibit CD16a cleavage in primary human NK cells. Our first Aim is to genetically modify human pluripotent stem cells to derive NK cells that express non- cleavable CD16a or lack ADAM17 expression. These cells will be evaluated for their anti-tumor functionality by in vitro approaches. Aim 2 is to evaluate the in vivo anti-ovarian cancer activity of NK cells engineered to block CD16a shedding. We will use a human xenograft model and ovarian cancer cell lines as well as primary tumor samples. Aim 3 is to determine the in vivo anti-ovarian cancer activity of a selective ADAM17 inhibitor in combination with adoptively transferred human NK cells and therapeutic mAbs. The impact of our study is that it investigates CD16a shedding by NK cells as a pivotal mechanism of resistance to therapeutic mAbs in ovarian cancer patients.